drm/mediatek: Add fifo_size into rdma private data

Get the fifo size from device tree
because each rdma in the same SoC may have different fifo size

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>